In a network where a power line is used as a communication medium, information is carried on the power line. Therefore, when network communication through the power line is used in an office or an apartment house in a business complex or an apartment, there is a problem that an unintended terminal joins the network and intercepts communication, and hence security should be reinforced.
Regarding a technique for reinforcing security, for example, Japanese Patent Laying-Open No. 2002-325079 (Patent Document 1) or Japanese Patent Laying-Open No. 2004-222312 (Patent Document 2) discloses a technique for encrypting communication by sharing an encryption key (house code).
In addition, Japanese Patent Laying-Open No. 2006-166273 (Patent Document 3) discloses a management method for reinforcing security such that a head end portion (corresponding to CCo in the present invention) provides an encryption key to an authorized terminal device to prevent connection of an unauthorized terminal.
Meanwhile, Japanese Patent Laying-Open No. 2004-194264 (Patent Document 4) discloses a technique for permitting automatic plug-in of new equipment to a network when the new equipment is added to a network system.    Patent Document 1: Japanese Patent Laying-Open No. 2002-325079    Patent Document 2: Japanese Patent Laying-Open No. 2004-222312    Patent Document 3: Japanese Patent Laying-Open No. 2006-166273    Patent Document 4: Japanese Patent Laying-Open No. 2004-194264